The present invention relates to the use of an altered herpes simplex virus that is capable of killing tumor cells. More specifically, the present invention relates to a mutated, replication-competent Herpes Simplex Virus-1 (HSV-1) which contains mutations in two genes, is hypersensitive to antiviral agents such as acyclovir, is not neurovirulent and does not replicate in non-dividing cells, yet can kill nervous system tumor cells.
Malignant tumors of the nervous system usually are fatal, despite many recent advances in neurosurgical techniques, chemotherapy and radiotherapy. In particular, there is no standard therapeutic modality that has substantially changed the prognosis for patients diagnosed with malignant brain tumors. For example, high mortality rates persist in malignant medulloblastomas, malignant meningiomas and neurofibrosarcomas, as well as in malignant gliomas.
Gliomas are the most common primary tumors arising in the human brain. The most malignant glioma, the glioblastoma, represents 29% of all primary brain tumors, some 5,000 new cases per year in the United States alone. Glioblastomas are almost always fatal, with a median survival of less than a year and a 5-year survival of 5.5% or less. Mahaley et al., J. Neurosurg. 71: 826 (1989); Shapiro, et al., J. Neurosurg. 71: 1 (1989); Kim et al., J. Neurosurg. 74: 27 (1991). After glioblastomas are treated with radiotherapy, recurrent disease usually occurs locally; systemic metastases are rare. Hochberg et al., Neurology 30: 907 (1980). Neurologic dysfunction and death in an individual with glioblastoma is due to the local growth of the tumor.
In the past, viruses have been tested for their ability to treat various types of tumors in animals or humans. The proposed therapeutic mechanisms of viral cancer therapy in the prior art includes: (i) producing new antigens on the tumor cell surface to induce immunologic rejection, a phenomenon called "xenogenization", and (ii) direct cell killing by the virus, called oncolysis. Austin et al., Adv. Cancer Res. 30: 301 (1979); Kobayashi et al., Adv. Cancer Res. 30: 279 (1979); Moore, Progr. Exp. Tumor Res. 1: 411 (1960). Treatments for tumors in both animals and in humans have been based on wild-type virus, passage attenuated virus, or infected cell preparations. Kobayashi, Adv. Cancer Res. 30: 279 (1979); Cassel et al., Cancer 52: 856 (1983); Moore, Prog. Exp. Tumor Res. 1: 411 (1960).
Several animal models and animal tumors have been used to study oncolysis with wild-type viruses. Moore, Ann. Rev. Microbiol. 8: 393 (1954); Moore, Progr. Exp. Tumor Res. 1: 411 (1960). At least nine viruses have been shown to be capable of inducing some degree of tumor regression in a variety of tumors in mice, rats, rabbits, and guinea pigs. A major drawback seen in these early animal studies, however, was systemic infection by the virus.
To avoid systemic infection, the genetic engineering of viruses for use as antineoplastic agents has focused on generating altered viruses that are not capable of replication in non-dividing cells. Viruses capable of replication in dividing cells preferentially infect rapidly dividing tumor cells because they are incapable of replicating in non-dividing normal cells.
The use of replication-incompetent or defective viruses, which require helper virus to be able to integrate and/or replicate in a host cell, was hoped to prevent damage to non-tumor cells. The replication-defective herpes simplex virus vector system consists of an amplicon plasmid which, in herpes simplex virus infected cells, is replicated and packaged into viral particles. Defective herpes simplex virus vectors require helper virus to generate a herpes simplex virus vector.
The use of replication-defective retroviruses for treating nervous system tumors requires producer cells and has been shown to be limited because each replication-defective retrovirus particle can enter only a single cell and cannot productively infect others thereafter. Because these replication-defective retroviruses cannot spread to other tumor cells, they would be unable to completely penetrate a deep, multilayered tumor in vivo. Markert et al., Neurosurg. 77: 590 (1992).
Clinical trials employing retroviral vector therapy treatment of cancer have been approved in the United States. Culver, Clin. Chem 40: 510 (1994). Retroviral vector-containing cells have been implanted into brain tumors growing in human patients. Oldfield et al., Hum. Gene Ther. 4: 39 (1993). These retroviral vectors carried the HSV-1 thymidine kinase (HS-tk) gene into the surrounding brain tumor cells, which conferred sensitivity of the tumor cells to the anti-herpes drug ganciclovir. Of eight patients with recurrent glioblastoma multiforme or metastatic tumors treated by stereotactic implantation of murine fibroblast cells producing retroviral vectors, five patients demonstrated some evidence of anti-tumor efficacy but none were cured. Culver, supra (1994). Some of the limitations of current retroviral based therapy as described by Oldfield are (1) the low titer of virus produced, (2) virus spread limited to the region surrounding the producer cell implant, (3) possible immune response to the producer cell line, (4) possible insertional mutagenesis and transformation of retroviral infected cells, (5) single treatment regimen of pro-drug, ganciclovir, because the "suicide" product kills retrovirally infected cells and producer cells and (6) the bystander effect limited to cells in direct contact with retrovirally transformed cells. Bi, W. L. et al., Human Gene Therapy 4: 725 (1993).
In the early 1990's, the use of genetically engineered replication-competent HSV-1 viral vectors was first explored in the context of antitumor therapy. Martuza et al., Science 252: 854 (1991). A replication-competent virus has the advantage of being able to enter one tumor cell, make multiple copies, lyse the cell and spread to additional tumor cells. A thymidine kinase-deficient (TK.sup.-) mutant, dlsptk, was able to destroy human malignant glioma cells in an animal brain tumor model. Martuza, supra (1991). Unfortunately, the dlsptk mutants were only moderately attenuated for neurovirulence and produce encephalitis at the doses required to kill the tumor cells adequately. Markert et al., Neurosurgery 32: 597 (1993). Residual neurovirulence, as evidenced by a 50% lethality of intracranially-administered, replication-deficient herpes simplex virus viral vectors at 10.sup.6 plaque forming units (pfu) limits the use of such vectors for tumor therapy. Furthermore, known TK.sup.- HSV-1 mutants are insensitive to acyclovir and ganciclovir, the most commonly used and efficacious anti-herpetic agents.
Therefore, it remains of utmost importance to develop a safe and effective viral vector for killing tumor cells. Even though various attempts have been made to engineer a viral vector able to kill human tumor cells in vivo, no viral vector has provided attenuated neurovirulence at the dose required to kill tumor cells while exhibiting hypersensitivity to antiviral agents and an inability to revert to wild-type virus. Currently, no viral vector has been demonstrated to meet these criteria.